Process of classifying metal powders



March 20, 1945. J. WULFF 2,371,665

PROCESS OF. CL'ASSIFY'ING METAL POWDERS Filed Nov. 6, 71941 FURNACE.

I WATER QUENCH TANK STAMP MILL. 6

BALL MILL 7. I

6 scaszfl "mu-w aunwea I MAME-m: semen-rod I] I2 ARBIDE ow 15 J c S DER I I DECARBURIZING I 1 (names) METALLIC CONSTITUENTS Hz mou, ALLOY POWDER (Low CARBON) effective iron powders and iron alloy from c Patented Mar. 20, 1945 raocns s F CLASSIFYING METAL POWDERS John wulo, Cambridge, Mass. Application November 6, 1941. Serial No. 418,106

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This invention relates to an improved process of classifying metal powders.

The present application is a continuation in part of the prior application of John Wulfr Serial No. 338,826 filed June 4, 1940.

The present invention relates to the production and classification of iron powders. I

,One of the most recent and interesting developinents in thefield of metallurgy is the development of powder metallurgy, that is the fabrication of articles from metal powders. From a beginning innon-ferrous powderaas for example in the production of the so-called oilless bearings from copper powder, the art has extended rapidly and now includes the fabrication of articles, such as machine elements, from iron powders. While the field of powder metallurgy" is expand-ing rapidly its extension is restricted by a number of factors important among which is the present high cost of the starting material that is the pure iron powder. In the past such iron powders were produced fromspecial raw materials, for example electrolytic iron and the like which in their nature imposed a high initial cost on the raw material. An object of the present invention is to broaden the potential field of use of iron powders for powder metallurgy purposes by producing highly and iron. alloy source material. j The invention comprehends the concept of utilizing a cheap source of iron, such as relatively high carbon pig iron, low in sulphur and phosphorus, producing-a-heat from such material by conventional equipment to produce a white iron properties.

' ing.

' mentite or pearlite.

shot, i. e. a shotted material low in austenite and. high in pearlite, cementite and/or martensite.

This material is then disintegrated to produce in effect two major, phases or constituentathat is a carbon-rich phase or cementite and a carbonpoor phase, namely pearlite and the like.

According to the present invention these metallurgical phases are effectively segregated by subjecting the powdered material to a special type of magnetic separation. Considered briefly this separation step comprises heating the magraphite.or a suitable ceramic.

terial to raise the temperature to the Curie point 7 1 of one of the constituents and then passing the material through 'a magnetic separator. It will be appreciated that this novel method is applicable to the classification of any. mixture of- In the illustrative embodiment it is utilized to separate a mixture of relatively nonmagnetic carbide particles from relatively magnetic low carbon iron or iron alloy particles, such as pearlite. In such circumstances the cementite phase can be sharply separated or fractionated by heating the powder mixture to a temperature of substantially 215 C. and passing the heated material through the magnetic separator. The separated fractions may then be treated to modify their physical or chemical characteristics to the desired degree. I

In order more fully to explain the invention a typical process of producing metallurgical powders is illustrated in the In carrying out the improved process a white cast iron powder is first produced and, as explained, this is subjected to magnetic separation at the Curie point of the less magnetic component, namely, the carbides so that the carbides are sharply fractionated from the ceraw material, such as a suitable pig iron low in sulphur, phosphorus and silicon, is charged'to a suitable furnace I. As will be understood this furnace may be of any suitable type and for purposes of illustration is shown as an electric fur- Depending upon the particular desired analysis of the ultimate ferrugihous powder, alloy addition agents may be introduced into the melt. In order to simplify the description of the invention a simple procedure will be described, namely the production of a low carbon iron from high carbon iron raw material.

The charge in furnace I may be heated to the desired elevated temperature, say of the order of from about 1430 C. to about 1510 C. The heat is then transferred to ladle 2 and is poured through the refractory screen 3 constructed .of

This screen serves to divide the molten metal into streamlets. Such streamlets are immediately quenched and shotted by means of water or other suitable quenching medium forced through the jets 4. Such quenching and shotting transforms the molten'mass to solid shot of the order of /4 to i: of an inch or finer. Manifestly other specific methods of shotting and quenching may be utilized as for example by utilizinga ceramic stream divider and steam Je metallic powder constituents, the componentsof 'whichare characterized by differential mignetic- The shottedmaterial collects in the quench tank 5. This material is essentially a white cast iron and because of its high percentage of cementite, pearlite and martenslte is characterized flow sheet on the draw a As shown on the drawing sired temperature bon in the shot and this material therefore will be very hard. This-brittleness, as will be appreciated, will render the product amenable to rapid and effective disintegration in standard disintegrating equipment.

In the next step of the process the brittle shot is broken down into powder form comprised essentially of carbon-poor particlesyi. e." particles rich in pearlite and carbon-rich particles, i. e. particles rich in iron carbide. In' a. typical method, as shown, the shot is first broken down in a suitable mill, such as the stamp mill 6 and-is then further reduced in size by treatment in the .ball mill I. The product discharged from the ball-mill may be classified by passing over. the

screen 8. Particles above a predetermined size may berecycled through the ball mill through line 9. It will be understood that in these reduction steps the brittle cementite will crush readlly and will also assist in crushing the more metallic pearlite, troostite and undecomposed' austenite. '01 the shot? The product passing through the screen 8 which is to be treated according to the invention may'comprise a powder of from about 100 to 200 mesh.

Due to theinitial high carbon content of the raw material and'of the quenching. conditions this product isv comprised preponderantly of cemeritite or more metallic constituents anda definite percentage of iron carbide. The pearlite which contains .of'the order of 6.6 C. In order to sharply fractionatethe two constituents this magnetic characteristic is utilized to, the fullest degree, that is to say the difierential magnetic characteristics'are accentuated or increased by. 40

heating the mass to-the point of least magnetism oi the carbides, i. e. the Curie point, and while at this .point the mass is subjected to magnetic separation. f

, Since Curie point of carbidesis 215 G.) 15 inthe preferred embodiment the fine powder mass isheated substantially to this point.

The heating of the powder mass to the desired optimum separating temperature may be achieved by any suitable .emcient means.

purely illustrative example, as. shown in the drawing, the fine powderscreenings from screen I are ied to one end of a cylindrical container l0 andare transported'therethrough by means of ascreen conveyor. material is heated up-tovthe desired temperature by meansof the burner ll. ln lieu of this the. powder may be contacted in a tower or'other vessel by hot gases to: secure the desired increase in temperature. If desired theheating medium 0$ may be controlledthermostatically so as to insure a close control ofrthe temperature of the powder. l c

After the powder has been heated to the dethe magnetic separator l2. The magnetic constituent is sharply fractionated from the car bide'tailings. The carbide. powder, may: be sold as. such as a blasting powder or may be treated l e decarburizing furnace to reduce the carbon 7 content to any desired low degree. The pearlite screenings may be admixed with reduced iron During this passage the in heate? Ill it is passed to 5 beneficial for powders which areto be employed for compacts in that in the decarburization the pearlite and cementite constituents are largely decomposed to give a surface skin of the more plastic ferrite. I

It will be appreciated that the described p'roc-' ess provides a simple and eflicient method of sharply separating the more metallic low-carbon constituents from a mixture of these with high carbon constituents such as iron carbide. Such fractionation in turn renders particularly effective a method of producing low carbon" ironpow-- ders from cheap source material, such as pig iron -or high carbon scrap. Hence while a preferred embodiment of the invention has been described it is to be understood that this is given to exemplify the underlying principles of the invention and not as limiting it tothe particular embodi-' ment chosen for illustration.

I claim: l.( A method of producing classified iron powder which comprises forming a white cast iron shot, disintegrating the shot to a powder to form g carbon-rich and carbon-poor components and .is much more magnetic than the iron, carbide- 35 powder which comprises forming a. high carbon shot of the white castfiron type. disintegrating the hot to a predetermined particle size to form pear ite-rich particles and carbide-rich particles and magnetically separating the carbiderich particles at the Curie point of the carbiderich'particles.

3. A method of producing low carbon iron- -containing powder which comprises forming a high carbon shot of the white castiron type. disintegrating the shot to a predetermined particle size to' form pearlite-rich and carbide-rich particles, heating the powder to a temperature of V substantially 215 -C. and passing the powder through amagnetic separator to separately recover the pearlite rich and carbide-rich iractions.

4. A method of producing low carbon ironcont'aining powder which comprises jorming a high carbon shot of the white cast iron-type, disintegrating the shot to a predetermined size to form iron-rich and carbide-rich particles, heating the powder to a temperature of substantially 215 C. and passing the'heated powder through a magnetic separator;

' 5. A 'method of producing-low carbon ironcontainingpowder I which comprises forming a high carbon shot of the white cast iron type low Such decarburizing treatment is especially in graphitizing elements, disintegrating the shot to apredetermined size .to form iron-rich and carbide-rich particles, heating the powder to the Curie point of the carbide-rich particles and Curie point.

1 JOHN magnetically separating the 'particles at such 

